1. Field of the Invention
The present invention generally relates to a fixing device to fix an image on a sheet of recording media, and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions, including the fixing device.
2. Discussion of the Background Art
Image forming apparatuses, such as, printers, facsimile machines, plotters, or multifunction machines including at least two of these functions, typically include a fixing device to fix toner images on sheets of recording media with heat and pressure (e.g., a heat-type fixing device). Fixing devices generally include a fixing member, a heat source configured to heat the fixing roller, and a pressing member configured to press against the fixing member. There are roller-fixing type fixing devices, in which both the fixing member and the pressure member are rollers, and belt-fixing type fixing devices, in which at least one of the fixing member and the pressure member is an endless belt. In fixing devices, the toner image on the sheet is fused with the heat from the fixing, member and then is fixed with the pressure generated between the fixing member and the pressing member while the sheet passes through a so-called fixing nip, where the pressure member presses against the fixing member.
FIG. 1 is a schematic end-on cross-sectional diagram illustrating a known heat-type fixing device including a fixing roller 100 and a pressing roller 200 pressing against the fixing roller 100.
As shown in FIG. 1, a sheet P (e.g., a recording medium) on which a toner image T is formed passes through a fixing nip N and then is discharged in a direction indicated by alternate long and short dashed arrow a. However, in this type of fixing devices, the actual direction in which the sheet P is discharged from the fixing device (sheet discharge direction) tends to shift from the direction indicated by arrow a toward the fixing roller 100 because the fused toner T adheres to the fixing roller 100 and pulls the sheet P toward the fixing roller 100. If the adhesion force of the fused toner T to the fixing roller 100 causes the sheet discharge direction to shift across a threshold line indicated by line b (threshold line b) to one side (in FIG. 1, to the left), it can happen that the sheet P winds around the fixing roller 100.
More specifically, if F1 and F2 respectively represent the adhesion force of the fused toner T to the fixing roller 100 and a force required to bend the sheet P by an angle θ from the direction indicated by arrow a to the threshold line b to wind around the fixing roller 100, then the sheet P can be separated from the fixing roller 100 when a relation F1<F2 is satisfied. Put simply, the sheet P separates cleanly from the fixing roller 100 and is discharged properly so long as the force of adhesion of the fused toner T to the fixing roller 100 is not enough to overcome the sheet P's stiffness and wrap the sheet P around the fixing roller 100. The threshold line b is the border of the angle θ at which the sheet P, if bent, winds around the fixing roller 100.
In view of the foregoing, several approaches have been tried to facilitate separation of the sheet from the fixing roller. For example, releasing agent such as wax may be added to the toner to reduce the adhesion force of the toner. Alternatively, the diameter of the fixing roller is reduced as indicated by a chain double-dashed line shown in FIG. 1 to shift the threshold line b to a threshold line b′ shown in FIG. 1, thereby increasing the force F2 required to bend the sheet P (bending force F2) to the angle at which the sheet P winds around the fixing roller 100.
However, the above-described known approaches cannot prevent the sheet from winding around the fixing roller when the sheet is relatively thin and has a lower degree of stiffness because the bending force F2 required to cause the sheet to wind around the sheet is also smaller. Additionally, although the sheet discharge direction may be adjusted by a guide member, the image may be disturbed and/or the sheet may wrinkle if the image face (that side of the sheet on which the toner image is formed) of the sheet contacts the guide member.
Therefore, there is a need for a fixing device capable of facilitating separation of the sheet from the fixing member as well as preventing disturbance of images and/or creation of wrinkles on the sheet, which known approaches fail to do.